1. Field of the Invention
The present invention relates to a miniaturized electronic circuit module to be mounted on other wiring boards (motherboards) and a method for fabrication thereof.
2. Description of Related Art
Since mobile information devices require miniaturized and high-density circuitry, miniaturized modules incorporated in such devices have to realize sophisticated functions within a space as small as possible. To achieve this, many modules are multichip modules in which, on a single module board provided with terminals for external connection, a core LSI (large-scale integration) chip is bare-chip mounted and various peripheral components are mounted. FIGS. 14 and 15 show an exemplary configuration of a multichip electronic circuit module 50 (hereinafter also referred to simply as a “module 50”). FIG. 14 is a vertical sectional view of the module 50 mounted on a motherboard 60, as seen on a plane perpendicular to the motherboard 60, and FIG. 15 is a perspective view showing only the single module 50 as seen from the bottom face thereof (the face in contact with the motherboard).
First, as shown in FIG. 14, on the front face of a multilayer module board 51 on which the module 50 is built, surface-mounted components such as peripheral chips 56, including a resistor or a capacitor, and an IC 57 in a general-purpose package are mounted by reflow soldering, for example. In this figure, these surface-mounted components are covered with a metal shield case 58.
On the other hand, as shown in FIG. 15, in the center of the bottom face of the module board 51, there is formed a quadrangular cavity 52, inside which a core LSI chip 53 is die-bonded in a bare-chip state. Connecting terminals on the LSI chip 53 are electrically connected to lands formed on the surface of the cavity 52 by wire bonding with gold wires 54. The cavity is then sealed with resin 55. JP-A-2003-060523 discloses the above-described configuration wherein, on the bottom face (the face in contact with the motherboard) of the module board 51, there is formed the cavity 52, inside which a semiconductor chip is provided.
Now, the module 50 as described above has electrode terminals (connecting terminals) 59 in peripheral portions around the cavity 52 formed on the bottom face of the module board 51. Through the electrode terminals 59, the module 50 is mounted on module-mounting lands 61 formed on the motherboard 60. Mounting is performed by such method as reflow soldering, whereby electrical connection with the outside of the module is established.
In many modules, the electrode terminals 59 are formed over the mutually continuous bottom and side faces of the module board 51. This is because it is thereby possible to form solder fillets (solder wicking portions) 62 (see FIG. 14) on the side faces of the module 50 that enhance soldering strength and allow the easy checking of solder connections.
Incidentally, the above-described electrode terminals 59 formed over the mutually continuous bottom and side faces of the module: board 51 are generally formed by the following method. As shown in FIG. 16, on cut margins 63 provided on a sheet circuit board 51a, which will eventually be cut into individual unit modules 50, through holes 64 having a width wider than that of the cut margins 63 are formed. Then, after the inner walls of the through holes 64 are plated with a conductive material, the cut margins 63 are cut away in such a way that the inner walls of the through holes 64 are left on the side faces of the module board 51.
Used as the above-described module board 51 having a cavity for fabrication of the module 50 is, in general, an LTCC (low temperature co-fired ceramic) board or a build-up multilayer board made of resin. In production of such a board, the employed method is laying thin layers one on top of another from which a portion corresponding to the cavity 52 is removed, or laying thin layers having no cavity 52 one on top of another, followed by the hollowing of the resultant layers to obtain the cavity 52. This makes production of such a board complicated as compared with a conventional flat multilayer board having no cavity, and thus leads to a longer production lead time and higher production cost.
Moreover, such a structure as that having a cavity has various structural limitations concerning, for example, the strength of the board or the interference between a device and the internal surfaces of the cavity 52 when the device is actually mounted inside the cavity 52. This limits module design flexibility.
As mentioned earlier, a plurality of unit module boards 51 are formed on a single sheet circuit board 51a, and then the sheet circuit board 51a is cut into individual unit module boards 51. In this case, however, the cavities 52 formed on one side of the sheet circuit board 51a cause warpage of the sheet circuit board 51a. This leads to lower dimensional accuracy of the cut module boards 51.
Furthermore, mounting of electronic components on the module board 51 has to be performed to the sheet circuit board 51a to improve the mounting efficiency and thereby reduce the production cost. However, the lower dimensional accuracy of the sheet circuit board 51a due to the warpage thereof just mentioned above makes it difficult to perform mounting to the sheet circuit board 51a. Thus, in most cases, the sheet circuit board 51a is cut into individual unit module boards 51, and then electronic components are mounted thereon. This is highly inefficient.
In addition to this, as described above, the connecting terminal 59 is formed over the mutually continuous bottom and side faces of the module board 51. This requires complicated process and thus leads accordingly to lower production efficiency.
In view of the conventionally experienced problems described above, it is an object of the present invention to provide a miniaturized electronic circuit module that does not suffer from the problems of the conventional miniaturized electronic circuit module having a cavity formed therein, and that can realize sophisticated functions within a space as small as possible, and to provide an efficient method for fabrication thereof.